The present invention relates to a grease composition to be enclosed in various rolling bearings.
The trend is for more motors for use in fan for air conditioner and HDD spindle to be reduced in its size and output for the purpose of inhibiting the heat generation taking into account the recent environmental restrictions. Therefore, rolling bearings for these purposes have been required to meet sufficient torque characteristics as important characteristics.
The dynamic friction torque of a rolling bearing occurs due to the friction caused by minute slippage on the rolling contact surface, friction by sliding on the sliding contact surface in the bearing or viscosity resistance of grease. Among these factors, the viscosity resistance of grease is known to be affected by the dynamic viscosity of base oil and the viscosity of grease. Accordingly, the dynamic viscosity of base oil is attributed to the shearing resistance of lubricant developed when a fluid lubricant film is formed. Thus, the reduction of this dynamic viscosity is a great solution to the problem of reducing the dynamic friction torque of rolling bearing. Further, since the viscosity of grease has an effect on the channeling properties shown when the bearing is subject to shearing on the interior thereof during rotation, the reduction of the viscosity of grease is another effective solution.
However, when the dynamic viscosity of the base oil is reduced, the desired film thickness can hardly be secured because the fan motor for air conditioner for example may operate at a relatively low speed in an inverter control process. Further, a base oil having a low dynamic viscosity normally has a low heat resistance and thus causes a problem of acoustic durability. On the other hand, the reduction of the viscosity of grease requires the increase of the mixing proportion of a thickening agent, causing a relative reduction of the amount of base oil in the grease. Further, the increase of the mixing proportion of a thickening agent causes the grease to have an enhanced resistance to mechanical shearing. As a result, the amount of base oil supplied onto the lubricated surface of the bearing is reduced, making it impossible to invariably maintain the desired lubricating properties over an extended period of time.
Thus, the reduction of the dynamic viscosity of base oil and the viscosity of grease is limited. For the grease to be enclosed in the rolling bearing for the foregoing purpose, it is said preferred that the dynamic viscosity of base oil be from 10 to 500 mm2/s (40° C.), the viscosity of grease be in NLGI No. 2-3 grade, and the amount of thickening agent be from 5 to 20% by mass.
Under these circumstances, a grease having a lithium salt of aliphatic acid incorporated as a thickening agent in an ester oil as a base oil is enclosed in a motor having requirements for acoustic characteristics in particular. This is because an ester oil has a higher heat resistance than a mineral oil and has a polar group in its molecular structure. This polar group causes the mineral oil to enhance its adsorptivity to the surface of metal, improving the friction characteristics and hence the acoustic durability.
As mentioned above, although some effective methods for improving acoustic durability have been found, no grease compositions effective for the reduction of bearing torque were obtained.